Risk factors of developing contralateral breast cancer after first primary breast cancer treatment

Abstract Background Breast cancer (BC) is the most common cancer among women worldwide. Increased survival of primary BC (PBC) has increased contralateral breast cancer (CBC) and become a health problem. Aims This study aimed to determine the effect of disease‐free interval (DFI), risk factors and PBC characteristics on the progression of CBC within primary BC survivors. Methods and Results This retrospective study identified 5003 women diagnosed with breast cancer between 2000 and 2020 in the cancer research center. The study included 145 CBC and 4858 PBC survivors, with CBC diagnosed at least 6 months after the detection of primary BC. ER+, PR+, and HER2+ were reported in 72.13%, 66.67%, and 30% of CBC patients. Invasive ductal carcinoma (IDC) BC was reported in 69.57% of patients, and 81.90% and 83.64% of the patients were treated with adjuvant chemotherapy and external radiotherapy. The Kaplan–Meier method indicated that the median time interval between PBC and CBC was 3.92 years, and the 5‐year DFI was 97%. The Cox proportional hazard regression model indicated that although more than half of the participants had no family history of BC (69.57%), women 60 years and older were negatively associated with CBC. Conclusion This study provides the first investigation of CBC and DFI risk factors among PBC survivors in Iran. Age was found to be negatively associated with CBC development particularly after the age of 60, indicating the necessity of tracking CBC survivors carefully in this age group.